In known incremental position measuring instruments, typically a periodic scale graduation is scanned with a likewise periodic scanning structure, and the resultant signal is further used elsewhere as a measurement signal. In the ideal scanning situation, as exactly sinusoidal a signal as possible would be available. In such a situation, further signal subdivision can be done by known interpolation methods. However, for several reasons, usually the output signal shape departs from the ideal sinusoidal shape. For example, the division ratios of the scale graduation and scanning graduation may deviate from ideal. Also edge fuzziness in the scale graduation bars corrupts the resultant signal. As a general rule, the signals to be evaluated thus have harmonic components. In order to have exact positional determinations in incremental position measuring instruments, however, the most harmonic-free scanning signals as possible are necessary.
A number of proposals have been disclosed to attempt to obtain the most harmonic-free measurement signals in incremental angle and length measuring systems. One proposal involves filtering the harmonic components of the measurement signal. The filtering technique always involves the use of a specially designed scanning plate to create the most optimized possible superposition of various signal components, phase-displaced from one another. In a number of applications, only certain harmonics are eliminated since filtering out all the existing harmonic components would greatly impair the degree of modulation of the fundamental. The phase-displaced signal components required to eliminate harmonic components are generated at various scanning sites on the sides of the scanning plate.
The necessary extent or length of the scanning area for the desired filtration is called the filter length. In the normal situation, it extends in the measuring direction, or in other words in the direction in which a relative displacement takes place between a scale graduation and a scanning graduation.
European Patent Disclosure No. EP 157 177 describes a so-called adiabatic filter arrangement in which inside the scanning area of the scanning plate a "slow change" in the center position of the bars over the complete scanning field is designed in accordance with a predetermined function. The complete filter length is typically an order of magnitude of 10 to 100 graduation periods of the scale graduation or scanning graduation.
European Patent Disclosure No. EP 645 607 describes a mixed filter arrangement. Here the sequence of bar displacements of an adiabatic filter, ordered normally in accordance with the above-explained function, are mixed by a deterministic or random method. The resultant effective filter length is approximately 3 to 10 graduation periods.
European Patent Disclosure No. EP 541 828 describes a short-period filter arrangement. This includes a period arrangement of a group with only a few bars, with the center positions of the bars and/or the widths of the bars being varied within the group such that a filtration of certain harmonic filter components is attained, typically odd-numbered harmonics. In this filter arrangement, the filter length is identical with the period of the intended group of lines and is approximately 3 to 5 graduation periods, depending on the filter version chosen.
Filters with more than one bar per graduation period are also known, for example from European Patent Application No. EP 95 109 701.3, not yet published, which is assigned to the present assignee. In the case of large or coarse graduation periods, a cosine transmission function of the scanning plate, which is required for optimal filtration of harmonics, is approximated by a plurality of fine bars within one graduation period. The resultant filter length accordingly amounts to one division.
With the aid of the scanning plate designed by these principles, the harmonic content of a periodic pattern of stripes, which is generated by a periodic scale graduation on the scanning plate, is filtered. However, because of given structural features, the scanning plate must be positioned in the beam path of the position measuring instrument upstream of the scale graduation; that is, the beams emitted by a light source with appropriate optics in front of it first pass through the scanning plate and only after that reach the scale graduation. In such arrangements, the term inverse scanning order will be used. Such inverse scanning arrangements also occur in incident-light position measuring instruments, for example, in which once again the corresponding beams pass first through the scanning plate and then project an intensity pattern onto the scale graduation. Scanning arrangements are also known in which a displaceable scale graduation is projected by a scanning graduation onto a second scanning graduation; that is, two scanning graduations are provided.
In the case where there are very small scanning distances between the scale graduation and the scanning graduation and in which an intensity distribution on the scale or a further scanning grating is determined solely by the shadow cast by the scanning plate, each of the aforementioned filter arrangements can in principle also be used in other scanning arrangements, for example in the aforementioned inverse scanning arrangements. However, if the scanning distances increase, then the intensity distribution on the scale is caused not only by the resultant cast shadow, on the contrary, diffraction phenomena are also involved, which impair or eliminate the filtering action of the filter arrangements known thus far. The reason for this is the phase displacements that the beams of light undergo in propagation between the various gratings, so that different phase displacements result for the various orders of diffraction. Accordingly, the maximum allowable scanning distance is directly dependent on the filter length chosen. The greater the filter length is, the greater the allowable or tolerable scanning distance range. Of the filter arrangements discussed above, it is therefore only the adiabatic filter arrangements that are usable for most average scanning distances. Long filter lengths, on the other hand, have still other disadvantages, since local soiling and/or local discontinuities of the scale graduation each cause a major phase displacement of the scanning signals, which is dependent on the scale position. This is because the superposition and hence the balance of the phase-displaced signal components of the fundamental are interfered with. As a consequence, there are major errors in the ensuing interpolation of the output signals.
In most known position measuring instruments, it is necessary merely to filter out certain harmonic signal components. The odd-numbered harmonic signal components (n=3, 5, 7, . . . ) have an especially disturbing effect on the output signals. The even-numbered harmonic signal components (n=2, 4, 6, . . . ) can normally be largely suppressed by known differential switching of push-pull signals. Moreover, suppression of the even-numbered harmonic signal components results in a known manner because of the embodiment of the scale or scale graduation, if the scale or the scale graduation has a line width that is on the order of magnitude of half the graduation period.
It is thus desirable to provide a device for filtering odd-numbered harmonic signal components in an incremental position measuring instrument that has the shortest possible filter length and which can be used in various scanning arrangements, for example, the aforementioned inverse scanning arrangements in which the light arriving from a light source first passes through the scanning plate and only then reaches a scale graduation. In addition, the desired filtering action should be independent of the scanning distance.